Two emission mechanisms in the Fermi Bubbles: A possible signal of annihilating dark matter

Abstract

We study the variation of the spectrum of the Fermi Bubbles with Galactic latitude. Far from the Galactic plane (|b|≳ 30°), the observed gamma-ray emission is nearly invariant with latitude, and is consistent with arising from inverse Compton scattering of the interstellar radiation field by cosmic-ray electrons with an approximately power-law spectrum. The same electrons in the presence of microgauss-scale magnetic fields can also generate the the observed microwave “haze”. At lower latitudes (|b|≲ 20°), in contrast, the spectrum of the emission correlated with the Bubbles possesses a pronounced spectral feature peaking at ∼1–4 GeV (in E2dN/dE) which cannot be generated by any realistic spectrum of electrons. Instead, we conclude that a second (non-inverse-Compton) emission mechanism must be responsible for the bulk of the low-energy, low-latitude emission. This second component is spectrally similar to the excess GeV emission previously reported from the Galactic Center (GC), and also appears spatially consistent with a luminosity per volume falling approximately as r−2.4, where r is the distance from the GC. Consequently, we argue that the spectral feature visible in the low-latitude Bubbles is most likely the extended counterpart of the GC excess, now detected out to at least ∼2–3 kpc from the GC. The spectrum and angular distribution of the signal is broadly consistent with that predicted from ∼10 GeV dark matter particles annihilating to leptons, or from ∼50 GeV dark matter particles annihilating to quarks, following a distribution similar to, but slightly steeper than, the canonical Navarro–Frenk–White (NFW) profile. We also consider millisecond pulsars as a possible astrophysical explanation for the signal, as observed millisecond pulsars possess a spectral cutoff at approximately the required energy. Any such scenario would require a large population of unresolved millisecond pulsars extending at least 2–3 kpc from the GC.